Conventionally, polyorganosiloxane compositions curing into silicone rubber have been well known, and are widely used as a potting material, a coating material, a molding material for shaping, injection molding and so on, and a covering material in an electric and electronic field, and optical and optoelectronics, sensor, architecture fields, utilizing its excellent properties such as weather resistance, heat resistance, electrical insulation, hardness, mechanical strength, elongation and so on. Among them, a polyorganosiloxane composition which cures by an addition reaction increases in usage in the above-described fields because it quickly cures by appropriate heating and releases no corrosive substance during curing.
Then, for increasing the productivity in the injection molding, it is necessary to remove the cured product after molding from the metal mold as quick as possible and to perform next molding. Therefore, a polyorganosiloxane composition forming a cured product excellent in mold release property with respect to the metal mold is desired.
As the polyorganosiloxane composition from which the cured product excellent in mold release property is obtained, a curing composition has been proposed which is made by mixing in advance an alkenyl group-containing polyorganosiloxane and a reinforcing filler, performing heat treatment or uniformly mixing them, then mixing a hydrogen atom-containing polyorganosiloxane and a platinum-based catalyst to the obtained compound, and further adding and mixing a polyorganosiloxane containing hydroxyl groups bonded to silicon atoms in a molecule as a component for improving the mold release property (refer to, for example, Patent Reference 1 (JP-A Hei 11-140319)).
Besides, a silicone resin composition has been proposed which contains, as essential components, an organopolysiloxane having two or more alkenyl groups in one molecule, an organohydrogenpolysiloxane having two or more hydrogen atoms bonded to silicon atoms in one molecule, a platinum-based metal catalyst, and a release agent such as a fatty acid ester of an erythritol derivative, and has excellent metal mold release property (refer to, for example, Patent Reference 2 (Japanese Patent No. 4697405)).
However, both of the compositions described in Patent Reference 1 and Patent Reference 2 are insufficient in mold release property of the cured product with respect to the metal mold. Further, the compositions have a problem in formation of a coating of silicone on the surface (molding surface) of the metal mold when molding is repeated, causing a decrease in quality of a molded product (lens or the like). Hence, it is necessary to wash the molding surface of the metal mold so as to remove the silicone coating, but the metal mold having a complicated shape is difficult to wash, and therefore a molding silicone composition never contaminating the metal mold is desired.
Moreover, as curable silicone compositions, a silicone composition has been proposed, as a curing silicone composition, which contains an alkenyl group-containing organopolysiloxane composed of a dialkylpolysiloxane having two or more alkenyl groups, on average, in a molecule and having a predetermined viscosity and an organopolysiloxane having a tetrafunctional siloxane unit and containing a alkenyl group at a predetermined rate as a main component, an organohydrogenpolysiloxane having a tetrafunctional siloxane unit and containing a silicon atom-bonded hydrogen atom at a predetermined rate, and a hydrosilylation reaction catalyst, and having a hardness of 30 or more and 80 or less and an elongation of 50% or more (in conformity with JIS K6253), or having a hardness of 75 or less and an elongation of 35% or more (refer to, for example, Patent Reference 3 (Japanese Patent No. 5475295) and Patent Reference 4 (Japanese Patent No. 5568240)).
However, the compositions described in Patent Reference 3 and Patent Reference 4 also have a problem in the cured product being poor in mold release property with respect to the metal mold and being likely to contaminate the metal mold.